Exhaust Line for Heat Engine

ABSTRACT

The invention relates to an exhaust line ( 10 ) for a heat engine, comprising a nitrogen oxide trap ( 28 ) which is installed on a filter section ( 24 ) between an exhaust inlet ( 12 ) and an exhaust outlet ( 14 ). The invention also comprises: a bypass section ( 26 ) which is disposed between the nitrogen oxide trap ( 28 ) and the exhaust inlet ( 12 ) in order to divert the exhaust gas away from the nitrogen oxide trap ( 28 ), and exhaust gas switching means ( 32, 24 ) which enable the gases to flow through the filter section ( 24 ) or the bypass section ( 26 ).

The present invention relates to an exhaust line for a heat engine, of the type comprising, between an exhaust inlet and an exhaust outlet, a nitrogen oxide trap which is fitted on a filtering portion.

Currently, specific motor vehicles comprise internal combustion engines which operate principally with a mixture of fuel and oxidant which is referred to as lean. A mixture of this type is characterised by an excess of oxidant relative to fuel, the mixture which supplies the engine being substoichiometric in terms of fuel. Conversely, an engine which operates under conditions which are superstoichiometric in terms of fuel is said to be supplied with a rich mixture.

Engines of this type are used owing to their reduced fuel consumption. However, they operate under conditions such that significant quantities of nitrogen-containing derivatives such as nitrogen oxides are produced. Nitrogen oxides of this type, generally referred to as NOx, are harmful to the environment and it is desirable to eliminate them.

To this end, the exhaust line of the motor vehicle is equipped with catalytic depollution means and a nitrogen oxide trap which is generally referred to using the acronym “NOx trap”.

As known per se, the nitrogen oxide trap adsorbs the nitrogen oxides on specific catalytic elements when the engine operates with a lean mixture. When the adsorption limit of the nitrogen oxide trap is reached, they are regenerated. To this end, the engine is controlled so as to operate with a rich mixture for a few seconds. A rich mixture is characterised by superstoichiometric conditions in terms of fuel, the mixture comprising an excess of fuel relative to the oxidant introduced.

Nitrogen oxide traps operate in a satisfactory manner in a range of temperatures of between 300° C. and 600° C. They are heated by the exhaust gases themselves. Therefore, the nitrogen oxide traps are arranged, along the length of the exhaust line, as close as possible to the engine.

However, this position on the exhaust line renders the nitrogen oxide traps incompatible with engines whose maximum operating temperatures may reach 900° C. Above 600° C., the catalytic material present in the nitrogen oxide trap is destroyed.

It was envisaged to develop catalytic materials which withstand very high temperatures but the results are not very satisfactory. It was also envisaged to arrange, upstream of the nitrogen oxide trap, a heat-exchanger which allows the temperature of the exhaust gases to be reduced before they pass through the nitrogen oxide trap. However, the cost of this solution is very high.

The object of the invention is to provide an exhaust line for a heat engine which comprises a nitrogen oxide trap whilst being compatible with a heat engine for which the outlet temperatures of the exhaust gases are greater than the temperatures which the catalytic elements of the nitrogen oxide trap are capable of withstanding.

To this end, the invention relates to an exhaust line for a heat engine of the above-mentioned type, characterised in that it comprises, between the nitrogen oxide trap and the exhaust inlet, a branching portion which is capable of branching the exhaust gases away from the nitrogen oxide trap and switching means for the exhaust gases in order to cause them to flow through the filtering portion or branching portion, and in that it comprises a three-way catalyser which is arranged between the inlet and the upstream portion of the filtering portion and the branching portion.

According to specific embodiments, the exhaust line comprises one or more of the following features:

-   -   the switching means comprise means for blocking the filtering         portion immediately downstream of the nitrogen oxide trap;     -   the switching means comprise a three-way valve which connects in         parallel the branching portion and the filtering portion to the         remainder of the line;     -   the exhaust line comprises a fork which is arranged between the         inlet and, on the one hand, the upstream portion of the         filtering portion and, on the other hand, the upstream portion         of the branching portion, and the three-way valve is arranged         downstream of the filtering portion and the branching portion         and is capable of selectively connecting them to a common         downstream portion of the exhaust line;     -   the exhaust line comprises, on the one hand, at least one sensor         selected from the group comprising a sensor for sensing the         position of at least one valve, a temperature sensor and a gas         composition sensor and, on the other hand, means for diagnosing         the operating state of the switching means based on the or each         sensor;     -   the exhaust line comprises means for controlling the switching         means, which control means comprise means for acquiring the         outlet temperature of the exhaust gases from the engine, and the         switching means are capable of generally branching the gases off         towards the branching portion when the temperature of the gases         is greater than 600° C.;     -   the exhaust line comprises means for controlling the switching         means, which control means comprise means for acquiring the         stoichiometric operating conditions of the engine, and the         switching means are capable of generally branching the gases off         towards the branching portion when the engine operates in a         state which is superstoichiometric in terms of fuel;     -   the control means are capable of periodically controlling the         switching means so as to temporarily cause the exhaust gases to         flow through the filtering portion during the phases in which         the exhaust gases generally flow through the branching portion;         and     -   the exhaust line comprises means for controlling the switching         means, which control means comprise means for acquiring the         future occurrence of a phase for operating the engine with a         lean mixture when it operates with a rich mixture, and the         control means are capable of controlling the switching means so         as to branch the gases off towards the filtering portion when         such an operating phase of the engine with a lean mixture is         detected.

The invention also relates to a propulsion assembly which comprises an engine which generally operates with a lean mixture and an exhaust line as described above.

The invention will be better understood from a reading of the following description, given purely by way of example and with reference to the drawings, in which:

FIG. 1 is a perspective view of an exhaust line according to the invention;

FIG. 2 is a schematic view of the exhaust line according to the invention with the valve in a first position which allows flow through the nitrogen oxide trap;

FIG. 3 is a flow chart which sets out in detail the control principle of the valve of the exhaust lines of FIGS. 1 and 2; and

FIGS. 4A, 4B, 4C are flow charts which set out in detail the diagnostic algorithms implemented in the exhaust line according to the invention.

The exhaust line 10 illustrated in FIGS. 1 and 2 is intended to be connected at the outlet of a heat engine which generally operates with a lean mixture.

The exhaust line thus comprises an inlet 12 which is capable of being connected to the exhaust cylinder head of the engine and an outlet 14 for discharging the exhaust gases into the atmosphere.

The inlet 12 is formed by a manifold 16. It is connected to the inlet of a three-way catalytic purification element which is known per se and which is generally referred to below as a three-way catalyser.

Downstream of the catalyser 18, the line comprises a tube 20 which is extended with a fork 22 which simultaneously supplies a filtering portion 24 and a branching portion 26 which are arranged in parallel. The filtering portion 24 comprises a nitrogen oxide trap 28.

The nitrogen oxide trap 28 is mounted as close as possible to the outlet of the engine along the length of the exhaust line so as to be brought rapidly to temperature. In specific instances, the portion of the exhaust line between the nitrogen oxide trap and the outlet of the engine is thermally insulated.

The branching portion 26 has no processing means at all. The two portions 24 and 26 are connected to each other in a downstream direction so as to open through an outlet tube 30 via a three-way valve 32 which is controlled by an actuator element 34. The outlet 14 is formed at the end of the tube 30.

As illustrated in FIG. 2, the three-way valve comprises a flap 33 which can be moved between a first position in which the filtering portion 24 is connected to the outlet portion 30, the branching portion 24 being insulated from the outlet portion 30, and a second position in which the branching portion 24 is connected to the outlet portion 30, the filtering portion 24 being insulated from the outlet portion.

The actuator 34 is connected to a control unit 36 for control thereof, which unit 36 receives information specific to the operation of the vehicle. In particular, a temperature sensor 38 is provided immediately downstream of the manifold 16. This is connected to the control unit 36.

Furthermore, the control unit 36 is connected to the unit 40 for controlling the operation of the engine which controls in particular the operating phases of the engine with a rich mixture or lean mixture.

The units 36 and 40 are capable of exchanging information and control commands. In particular, the unit 36 is capable of acting on the unit 40 so that it reduces the performance levels of the engine, so that it changes supply mode to a rich mixture or lean mixture or so that it brings about a regeneration phase.

Conversely, the unit 40 is capable of informing the unit 36 of the future initiation of a regeneration phase or of the supply mode of rich mixture or lean mixture being used.

Furthermore, the control unit 36 integrates a module 42 for diagnosing the operating state of the exhaust line.

According to a first embodiment, a sensor 44 for determining the position of the valve 32 is placed on the exhaust line and connected to the module 42.

In a variant, a temperature sensor 46 is arranged, in addition thereto or in place thereof, on the filtering portion 24 upstream of the nitrogen oxide trap 28.

According to still another variant, a sensor 48 for sensing gas composition, for example, a sensor for sensing the level of nitrogen oxides, is placed in addition thereto or in place thereof downstream of the valve 32 along the outlet portion 30.

The control unit 36 is suitable for implementing an algorithm for controlling the valve 32 and the control unit 40 of the engine as illustrated in FIG. 3.

During a start phase of the engine, at step 100, the valve 32 is positioned, at step 102, in a position for blocking the branching portion 26 and for opening the filtering portion 24. In this manner, regardless of whether the engine is in supply mode with a rich mixture or lean mixture, the nitrogen oxide trap 28 is heated under the action of the flow of the exhaust gases.

At step 104, it is determined whether the engine operates with a rich mixture. If this is not the case, it is determined, at step 106, whether a regeneration of the nitrogen oxide trap 28 is required. If this is the case, the unit 36 controls the unit 40 so as to control the engine temporarily with a rich mixture at step 106 in order to allow desorption and a reaction of the nitrogen oxides, as known per se.

If, at step 106, a regeneration is not necessary, or following the regeneration operation, step 104 and subsequent steps are implemented again.

If at step 104, the engine operates with a rich mixture, it is determined, at step 110, whether the temperature of the gases measured by the sensor 38 at the outlet of the engine is greater than 600° C. If this is not the case, step 104 and subsequent steps are implemented again.

If this temperature is greater than 600° C., the control unit 36 controls, at step 112, the switching of the valve 32, so that the filtering portion 24 is blocked and the branching portion 26 is, in contrast, placed in communication with the outlet portion 30.

At step 114, it is determined, from the processor 40, whether a change to a lean mixture is anticipated in the short term. If this is not the case, a delay time is initiated at step 116 and, as long as this delay time has not elapsed, step 104 and subsequent steps are implemented again.

Once this delay time has elapsed, the valve 32 is temporarily switched so that, for a shortened period of time, the exhaust gases flow through the nitrogen oxide trap 28, thus allowing it to increase in temperature so that it is maintained in a range of temperatures between 300° C. and 600° C.

Following this temporary switching phase, the valve 32 is returned into position at step 118 so that the gases flow through the branching portion 24 and step 104 and subsequent steps are implemented again.

If, at step 114, the control unit 36 detects an imminent requirement to change to a lean mixture to control the engine, the valve 32 is switched at step 120 so that the gases flow only through the nitrogen oxide trap 28 in order to increase the temperature thereof, in anticipation of the change to a lean mixture. Following a predetermined period of time, the valve is retained in position 32 and the mixture supplying the engine is modified in order to allow control with a lean mixture at step 122.

It will be appreciated that, with an exhaust line of this type, the nitrogen oxide trap never operates at a temperature greater than 600° C. and is constantly kept at a temperature between 300° C. and 600° C., allowing the exhaust gases to be processed at any time when the engine is supplied with lean mixture.

FIGS. 4A, 4B and 4C illustrate diagnostic phases which are permanently implemented by the diagnostic unit 42, it being understood that one or more of the phases illustrated in these Figures can be implemented jointly.

In FIG. 4A, the test 140 determines from the sensor 44 whether the valve 32 is blocked. If this is the case, the control unit 36 controls, at step 142, the unit 40 in order to adapt control of the engine, so that it does not produce nitrogen oxides and/or maintains the temperature of the gases at less than 600° C.

In the presence of a temperature sensor 46, at step 150 illustrated in FIG. 4B, it is determined whether the temperature upstream of the nitrogen oxide trap is greater than 600° C. If this is the case, the performance levels of the engine are reduced, at step 152.

If a sensor 48 for sensing the composition of the gases is provided at the outlet, it is verified, at step 160, whether the content of nitrogen oxides is greater than a reference value V_(ref). If this is the case, the unit 36 controls the unit 40 so as to operate the engine with a rich mixture, at step 162. Following this, the valve 32 is switched at step 164 into the position thereof so that the exhaust gases flow only in the branching portion 24.

Generally, in the case of a malfunction as set out in FIGS. 4A, 4B and 4C, the control of the engine is adapted for operation in downgraded mode, that is to say, operation with a rich mixture and/or limitation of the outlet temperature of the gases. Furthermore, the driver is informed by an indicator light on the dashboard of the vehicle.

In this manner, even in the event of a malfunction in the exhaust line, the nitrogen oxide trap is preserved and the exhaust gases released into the atmosphere have a low content of nitrogen oxide, even if the performance levels of the engine, in terms of consumption or power, are reduced.

In a variant, the three-way valve 32 and the fork 22 are interchanged so that the three-way valve is arranged upstream of the filtering and branching portions.

In a variant of one or other of the embodiments, the three-way valve is replaced by two separate two-way valves, one of the valves being installed on the branching portion, the other valve being installed on the filtering portion. 

1-10. (canceled)
 11. Exhaust line (10) for a heat engine, comprising, between an exhaust inlet (12) and an exhaust outlet (14), a nitrogen oxide trap (28) which is fitted on a filtering portion (24), between the nitrogen oxide trap (28) and the exhaust inlet (12), a branching portion (26) which is capable of branching the exhaust gases away from the nitrogen oxide trap (28), switching means (32, 34) for the exhaust gases in order to cause them to flow through the filtering portion (24) or branching portion (26), a three-way catalyser (18) which is arranged between the inlet (12) and the upstream portion of the filtering portion (24) and the branching portion (26), characterised in that it comprises means (36) for controlling the switching means (32), which control means (36) comprise means (38) for acquiring the outlet temperature of the exhaust gases from the engine, the switching means (32) being capable of generally branching the gases off towards the branching portion (26) when the temperature of the gases is greater than 600° C., and in that the control means (36) comprise: means (40) for determining when the engine is operating with a rich mixture; means (40) for determining whether the temperature measured by the temperature acquisition means (38) is greater than 600° C.; means (40) for commanding the switching means (32) to branch the gases off towards the branching portion (26) when the engine operates with a rich mixture and when the temperature measured by the acquisition means (38) is greater than 600° C.; and means (40) for commanding the switching means (32) to maintain the flow of the gases through the filtering portion (24) when the engine is not operating with rich mixture or when the temperature measured by the acquisition means (38) is less than 600° C.
 12. Exhaust line according to claim 11, characterised in that the switching means (32) comprise means for blocking the filtering portion (24) immediately downstream of the nitrogen oxide trap (28).
 13. Exhaust line according to claim 12, characterised in that the switching means comprise a three-way valve (32) which connects in parallel the branching portion (26) and the filtering portion (24) to the remainder of the line (10).
 14. Exhaust line according to claim 13, characterised in that it comprises a fork (22) which is arranged between the inlet (12) and, on the one hand, the upstream portion of the filtering portion (24) and, on the other hand, the upstream portion of the branching portion (26), and the three-way valve (32) is arranged downstream of the filtering portion (24) and the branching portion (26) and is capable of selectively connecting them to a common downstream portion (30) of the exhaust line.
 15. Exhaust line according to claim 11, characterised in that it comprises, on the one hand, at least one sensor selected from the group comprising a sensor (44) for sensing the position of at least one valve, a temperature sensor (46) and a gas composition sensor (48) and, on the other hand, means (42) for diagnosing the operating state of the switching means (32, 34) based on the or each sensor.
 16. Exhaust line according to claim 11, characterised in that the means (36) for controlling the switching means (32) comprise means (40) for acquiring the future occurrence of a phase for operating the engine with a lean mixture when it operates with a rich mixture, and in that the control means (36) comprise means for controlling the switching means (32) so as to branch the gases off towards the filtering portion (24) when such an operating phase of the engine with a lean mixture is detected.
 17. Exhaust line according to claim 16, characterised in that the control means (36) comprise means for, when the means (40) for acquiring the future occurrence of a phase for operating the engine with a lean mixture when it operates with a rich mixture do not detect such a phase, initiating a time delay following which the switching means (32) branch the exhaust gases off towards the filtering portion (24) for a shortened period of time.
 18. Line according to claim 11, characterised in that the means (36) for controlling the switching means (32) comprise means (40) for acquiring the stoichiometric operating conditions of the engine, and in that the switching means (32) are capable of generally branching gases off towards the branching portion (26) when the engine operates in a state which is superstoichiometric in terms of fuel.
 19. Exhaust line according to claim 11, characterised in that the control means (36) are capable of periodically controlling the switching means (32) so as to temporarily cause the exhaust gases to flow through the filtering portion (24) during the phases in which the exhaust gases generally flow through the branching portion (26).
 20. Propulsion assembly which comprises an engine which generally operates with a lean mixture and an exhaust line according to claim
 11. 21. Exhaust line according to claim 11, characterised in that the switching means comprise a three-way valve (32) which connects in parallel the branching portion (26) and the filtering portion (24) to the remainder of the line (10). 